In general, a drilling tool holder with a cutting insert, working within a closed hole, has chip discharge as an important factor, and requires having a structure which discharges chips well and minimizes vibration.
An insert type drilling tool holder has unbalance of cutting inserts since in general two cutting inserts of outer and center cutting inserts are disposed asymmetrically in a tool holder pocket. Moreover, a small sized insert type tool holder for drilling a small hole has difficulty in disposing the two cutting inserts in the pocket of the tool holder, and has a poor rigidity.
Furthermore, in a case boring, facing, and turning are required in a lathe additionally after the drilling, though a few tool holders are required additionally, a high working productivity can not be secured owing to additional inventory, turret, and holder setting time period, and an increased cycle time period, and particularly, fabrication of a product of a good precision, and maintenance of good quality become difficult.